Oil-film Interferometry Research Articles

Drag Induced by Flat-Plate Imperfections in Compressible Turbulent Flow Regimes

This paper presents the results of a coupled experimental and numerical study aimed at evaluating the influence of typical aircraft surface imperfections on the flat-plate drag production in fully turbulent conditions. A test campaign involving high-level measurement techniques, such as microdrag evaluation, near-wall laser Doppler velocimetry, and oil-film interferometry, has been carried out at several Mach numbers from 0.5 to 1.3 to quantify the impact of a large range of flat-plate imperfections. Forward-facing and backward-facing plain and chamfered steps of different heights have been studied. A whole numerical study, based on Reynolds-averaged Navier–Stokes computations, has been completed and used for validation purposes. Given the very small order of magnitude of the forces to be measured and calculated, the relative comparison between experimental and numerical outcomes is satisfactory. Even if some local discrepancies exist, results show an overall good agreement in the positioning of the different imperfection drag productions. Such investigations are of prime interest to determine industrialization tolerances or excrescence geometries offering the best compromises between manufacturing costs and aerodynamic performances.

New insight into flow development and two dimensionality of turbulent channel flows

The experimental conditions required for a turbulent channel flow to be considered fully developed and nominally two dimensional remain a challenging objective. In this study, we show that the flow obtained in a high-aspect-ratio channel facility cannot be reproduced by direct numerical simulations (DNSs) of spanwise-periodic channel flows; therefore, we reserve the term “channel” for spanwise-periodic DNSs and denote the experimental flow by the term “duct.” Oil film interferometry (OFI) and static pressure measurements were carried out over the range \(200 < Re_{\tau } < 900\) in an adjustable-geometry duct flow facility. Three-dimensional effects were studied by considering different aspect ratio (AR) configurations and also by fixing the AR and modifying the hydraulic diameter \(D_{\mathrm{H}}\) of the section. The conditions at the centerplane of the duct were characterized through the local skin friction from the OFI and the centerline velocity at four different streamwise locations and through the wall shear based on the streamwise global pressure gradient. The skin friction obtained from pressure gradient overestimated the local shear measurements obtained from the OFI and did not reproduce the same AR dependence observed with OFI. Differences between the local and global techniques were also reflected in the flow development. For the range of Reynolds numbers tested, the development length of high-aspect-ratio ducts scales with the duct full-height \(H\) and is around \(200H\), much larger than the values of around 100–150H previously reported in the literature.

Three measuring techniques for assessing the mean wall skin friction in wall-bounded flows

The present paper aims at evaluating the mean wall skin friction data in laminar and turbulent boundary layer flows obtained from two optical and one thermal measuring techniques, namely, laser-Doppler anemometry (LDA), oil-film interferometry (OFI), and surface hot-film anemometry (SHFA), respectively. A comparison among the three techniques is presented, indicating close agreement in the mean wall skin friction data obtained, directly, from both the OFI and the LDA near-wall mean velocity profiles. On the other hand, the SHFA, markedly, over estimates the mean wall skin friction by 3.5–11.7% when compared with both the LDA and the OFI data, depending on the thermal conductivity of the substrate and glue material, probe calibration, probe contamination, temperature drift and Reynolds number. Satisfactory agreement, however, is observed among all three measuring techniques at higher Reynolds numbers, Rex>106, and within ±5% with empirical relations extracted from the literature. In addition, accurate velocity data within the inertial sublayer obtained using the LDA supports the applicability of the Clauser method to evaluate the wall skin friction when appropriate values for the constants of the logarithmic line are utilized.

Multi-image oil-film interferometry skin friction measurements

The benefits of analyzing multiple interferogram images obtained using oil-film interferometry in order to determine wall shear stress are assessed. Both dual- and multi-image analysis approaches are implemented and compared to standard single interferogram approaches. Each of the analysis approaches is derived from the thin oil-film equation. To assess the different implementations, both experimental interferograms and simulated oil films are used. The simulations determine the oil-film height as a function of space and time by solving the thin oil-film equation subject to a known wall shear stress distribution, whereas experimental interferograms were obtained in a turbulent flat plate boundary layer. The results of analyzing these data with the three different analysis approaches suggest that they all work equally well when test conditions are approximately steady. For tunnels with long transients at startup or when multiple test conditions need to be measured in a single run, the multi-image approach is recommended.

Comparison of experiments and simulations for zero pressure gradient turbulent boundary layers at moderate Reynolds numbers

A detailed comparison between recent direct numerical simulation (DNS) and experiments of a turbulent boundary layer under zero pressure gradient at Re θ = 2,500 and 4,000 (based on the free-stream velocity and momentum-loss thickness) is presented. The well-resolved DNS is computed in a long spatial domain (Schlatter and Orlu in J Fluid Mech 659:116, 2010a), including the disturbance strip, while the experiments consist of single hot-wire probe and oil-film interferometry measurements. Remarkably, good agreement is obtained for integral quantities such as skin friction and shape factor, as well as mean and fluctuating streamwise velocity profiles, higher-order moments and probability density distributions. The agreement also extends to spectral/structural quantities such as the amplitude modulation of the small scales by the large-scale motion and temporal spectral maps throughout the boundary layer. Differences within the inner layer observed for statistical and spectral quantities could entirely be removed by spatially averaging the DNS to match the viscous-scaled length of the hot-wire sensor, thereby explaining observed differences solely by insufficient spatial resolution of the hot-wire sensor. For the highest Reynolds number, Re θ = 4,000, the experimental data exhibit a more pronounced secondary spectral peak in the outer region (y/δ 99 = 0.1) related to structures with length on the order of 5–7 boundary layer thicknesses, which is weaker and slightly moved towards lower temporal periods in the DNS. The cause is thought to be related to the limited spanwise box size which constrains the growth of the very large structures. In the light of the difficulty to obtain “canonical” flow conditions, both in DNS and the wind tunnel where effects such as boundary treatment, pressure gradient and turbulence tripping need to be considered, the present cross-validation of the data sets, at least for the present Re θ -range, provides important reference data for future studies and highlights the importance of taking spatial resolution effects into account when comparing experiment and DNS. For the considered flow, the present data also provide quantitative guidelines on what level of accuracy can be expected for the agreement between DNS and experiments.

Shear stress and particle removal measurements of a round turbulent air jet impinging normally upon a planar wall

When a round jet of air impinges normally upon a wall, it imposes a shear stress parallel to the wall in all directions from the impingement point. Particle removal from that surface is assumed to be mainly due to the imposed shear stress. This shear stress has been difficult to measure directly and has, in the past, been inferred from particle removal rates. Here we make a fundamental measurement of the mean shear stress imposed upon a planar wall by a normally-impinging turbulent air jet using the technique of oil-film interferometry. The resulting shear–stress distribution is then compared with measured removal rates of latex microspheres from a planar glass surface as a function of the radial distance from jet impingement normalized by the height of the nozzle exit above the surface. The particle removal experiments are carried out with sparse (few collisions) particle distributions. These experiments show that the efficiency of particle removal is directly but not linearly related to the imposed shear stress. A distinct shear stress threshold was found, below which little or no particle removal occurred.

Very high Reynolds number boundary layers over 3D sparse roughness and obstacles: the mean flow

Hot-wire and oil-film interferometry measurements are taken for 3D rough wall boundary layers at very high Reynolds numbers (61,000 < Re θ < 120,000) with low blockage ratios, 10 < δ/H < 135, and high roughness, 100 < H + < 4,900. The results cover flows over both rough walls and over obstacles and are compared with and provide extension to recent lower Reynolds number results. The validity of the Townsend ‘wall similarity hypothesis’ in relation to consistently increasing 3D roughness is interrogated. In agreement with recent work, Schultz and Flack (J Fluid Mech 580:381–405, 2007) and Castro (J Fluid Mech 585:469–485, 2007) found that, for relatively low roughness, Townsend’s hypothesis holds for the mean velocity field. With increasing roughness, the equilibrium layer diminishes and gradually vanishes. The viscous component of the wall shear stress decreases, while the turbulent component increases as the roughness effects extend across the boundary layer.

Empirical mode decomposition and Hilbert transforms for analysis of oil-film interferograms

Oil-film interferometry is rapidly becoming the preferred method for direct measurement of wall shear stress in studies of wall-bounded turbulent flows. Although being widely accepted as the most accurate technique, it does have inherent measurement uncertainties, one of which is associated with determining the fringe spacing. This is the focus of this paper. Conventional analysis methods involve a certain level of user input and thus some subjectivity. In this paper, we consider empirical mode decomposition (EMD) and the Hilbert transform as an alternative tool for analyzing oil-film interferograms. In contrast to the commonly used Fourier-based techniques, this new method is less subjective and, as it is based on the Hilbert transform, is superior for treating amplitude and frequency modulated data. This makes it particularly robust to wide differences in the quality of interferograms.

Drag Force Balance of a Blunt and Divergent Trailing-Edge Airfoil

M odifying the trailing edge of an airfoil can lead to significant improvements in its aerodynamic performance. Divergent trailing-edge (DTE) airfoils were first introduced to improve the liftto-drag ratio of supercritical airfoils at cruise conditions for commercial aircraft [1]. They are a natural evolution of Gurneyflaps used on low-speed airfoils to increase airfoil lift and reduce profile drag [2]. DTE airfoils are characterized by upper and lower surfaces that diverge from each other over about the aft 30% of chord forming a blunt trailing. This DTE approach was extended to low-speed applications such as wind turbine blades [3]. DTE profiles on wind turbine blades can improve aerodynamic performance and structural strength compared to profiles with sharp trailing edges. Critical to enhancing the aerodynamic performance of these airfoils and optimizing their geometrical configuration is the accurate characterization of surface flow properties as well as the precise determination of their aerodynamic characteristics. The profile drag of airfoils can be determined using three different approaches: dynamic force balance, wake surveys, and direct measurements of the two drag components, that is, skin friction and pressure drag. Direct measurements of drag components along the airfoil surface also provide useful information for characterizing the surface flow (i.e., the boundary-layer state). However, the accuracy of the available direct skin-friction measurement methods was previously questioned [4]. Recently, the oil film interferometry method that relies on local thinning of an oil film under shear stress has been proven to accurately measure skin friction [5]. In the present study, direct measurements of both surface pressure and skin friction are employed to 1) characterize the physics of the surface flow around a DTE airfoil, and 2) estimate accurately the profile drag from direct measurements and compare it with that estimated from a survey in the airfoil far wake.

An experimental investigation of a highly accelerated turbulent boundary layer

A canonical flat-plate turbulent boundary layer with Re θ = 4590 is exposed to a favourable mean streamwise pressure gradient sufficient to cause relaminarization. The favourable pressure gradient is generated by a linear contraction, yielding a peak value of the acceleration parameter of K = 4.4 × 10 −6 which is sustained for approximately 13 local boundary layer thicknesses. The relaminarization process is characterized by an extensive series of mean flow and turbulence measurements obtained at several representative streamwise locations. In anticipation of the loss of standard log-law behaviour, the local wall shear stress is directly measured using the oil-film interferometry technique. Mean flow measurements show a systematic variation in the Karman and additive constants with applied streamwise strain rate. The series of measurements also indicate an apparent decoupling of the outer and near-wall regions of the accelerating boundary layer. In accord with this, conditional measurements show that fourth-quadrant sweep events are virtually eliminated, while much less frequent but larger-amplitude near-wall second-quadrant ejection events remain. The reduction in fourth-quadrant sweep events is matched by an observed increase in near-wall third-quadrant events. The consequent reduction in near-wall Reynolds stress correlation and associated cross-stream momentum transport results in a large reduction in c f for the relaminarized flow.

Surface Shear Stress and Pressure Measurements in a Turbine Cascade

To further the understanding of secondary flow loss, a set of surface measurements is presented for a planar turbine cascade tested at low Mach number (maximum of 0.23) and at an inlet axial chord Reynolds number of 5.9×105. The endwall and airfoil surface measurements are of skin friction, limiting streamline direction, and static pressure. An oil film interferometry measurement technique applied to the endwall and airfoil surfaces (at some 2000 locations) provides an extensive passage surface map of skin friction values and limiting streamline topology. Measurements of pressures on the same passage surfaces are also presented, resulting in a complete picture of endwall and airfoil surface pressure and shear.

Refined cf relation for turbulent channels and consequences for high-Re experiments

There have been rising concerns regarding the accuracy of measurements in turbulent channel flows, in particular, measurements of the skin-friction results. In the present study, two different methods, namely, mean streamwise pressure gradient (PG) and oil film interferometry (OFI), are used to estimate the wall skin-friction relation, cf = f(Rem), for fully developed turbulent plane-channel flow over a wide range of Reynolds numbers. The channel skin-friction data are then fitted to the well-known logarithmic friction law, providing outstanding agreement with values for the constants of the logarithmic law of the mean velocity profile. A revised logarithmic skin-friction relation is developed, providing good agreement with our skin-friction results and data from the literature, when constants of the logarithmic friction relation adopted from the recent work of Zanoun et al (2003 Phys. Fluids 15 3079–89, 2005 4th Int. Conf. on Heat Transfer, Fluid Mechanics and Thermodynamics, HEAT2005, 19–22 September, Cairo, Egypt) are utilized. A new experimental channel facility is proposed, allowing measurements at high Reynolds numbers well beyond those achieved previously in laboratories, i.e. over five times the highest Rem reached in the present study, while maintaining sufficiently high spatial resolution.

Direct total skin-friction measurement of a flat plate in zero-pressure-gradient boundary layers

The total skin friction on a flat plate is directly measured by using a towing tank up to Reynolds number ReL ≃ 107 (or Rθ ≃ 104). Plates of 3.3 and 4.3 m in length are towed in still water, balancing the vertical weight by small flotation devices, and their drag force is measured by a highly sensitive load cell. We have developed a new technique to correct wave-making resistance, pressure resistance and drag on a turbulence simulator. When the measured total drag is converted into local drag, it is found that the local frictional resistance is about 6% smaller than that given by the Kármán–Schoenherr formula. But it is consistent with the values obtained by the floating element technique, oil film interferometry and asymptotic evaluations.

0220 乱流境界層における壁面せん断応力についての考察(OS2-4 乱流の計測・解析・制御,オーガナイズドセッション)

The purpose of this study is to evaluate the frictional resistance with sufficient accuracy and the drag coefficient at high Reynolds numbers. We have measured the resistance of flat plate with using towing tank. Correcting the wave-making resistance, pressure resistance, and drag on turbulence simulator, it is found that the measured frictional resistance is smaller than the Karman-Schoenherr formula. But it agrees with the values suggested by Oil-film interferometry in moderate Reynolds number.

Skin Friction Measurements Using Oil-Film Interferometry in NASA's 11-Foot Transonic Wind Tunnel

Skin-friction measurements were obtained on two laminar flow airfoils in the NASA Ames 11-ft transonic wind tunnel using oil-film interferometry. Improvements in the experimental technique allowed skin-friction measurements at two conditions per run in conjunction with performance testing, thus reducing the impact on the run schedule. Quantitative measurements of skin friction were used to determine transition location and the extent of separation (if any) on the airfoils providing much more insight into the flow than flow viz. The technique has been extended to obtain a quantitative measure of shear in the reversed flow region. The effects of various disturbances to the boundary layer were also investigated. Laminar flow was found as far back as 60% chord at a Reynolds number of 4.5 x 10 6 .

Flow structure and skin friction in the vicinity of a streamwise-angled injection hole fed by a short pipe

The velocity field and skin friction distribution around a row of five jets issuing into a crossflow from short (L/D ≃ 1) pipes inclined by 35° with respect to the streamwise direction, (i.e., “short holes”) are presented for two different jet supply flow directions. Velocity was measured using PIV, while the skin friction was measured with oil-film interferometry. The flow features are compared with previously published data for jets issuing through holes oriented normal to the crossflow and with numerical simulations of similar geometries. The distinguishing features of the flow field include a reduced recirculation region in comparison to the 90° case and markedly different in-hole flow physics. The jetting process caused by in-hole separations force the bulk of the jet fluid to issue from the leading half of the streamwise-angled injection hole, as previously reported by Brundage et al. (Tech Rep ASME 99-GT-35, 1999) and predicted by Walters and Leylek (ASME J Turbomach 122:101–112, 2000). The flow structure impacts the skin friction distribution around the holes, resulting in higher near-hole shear stress for a counter-flow supply plenum (jet fluid supplied by a high speed plenum flowing opposite to the free stream direction). In contrast, the counter-flow supply plenum was previously found to have the lowest near-hole wall shear stress for normal injection holes (Peterson and Plesniak in Exp Fluids 37:497–503, 2004b). Streamwise-angled injection generally reduces the near-hole skin friction due to the reduced jet trajectory resulting from the lower wall-normal jet momentum. Far downstream, the skin friction distributions are similar for the two injection angle cases.

Photogrammetry in Oil-Film Interferometry

Theapplication of photogrammetry to the measurement of skin friction using oil-film interferometry isdiscussed. Oil-film interferometry and photogrammetry are first described to justify the need for photogrammetry, and then the method is applied to two flows. The results indicate that when model curvature is high or the camera-to-model distanceisshort,largeuncertainties(oftheorderof10%)inskinfrictioncanbeintroducedifphotogrammetryisnot applied. For most applications demanding high-accuracy skin-friction measurements, approaches such as those described in this article should be considered. HEmeasurementofwallshearstressinaerodynamictestinghas increased significantly in the last decade. The increased use is primarily a result of the growth of image-based oil-film interferometry, which provides measurements of the mean wall shear stress that can be obtained relatively quickly and with good accuracy. The technique has been applied to simple laboratory flows as well as in large wind tunnels. Accompanying the increased use of oil-film interferometry has been a demand for higher accuracy and less time-intensive analysis. One way that these two issues can be addressedisthroughtheuseofphotogrammetry.Althoughthisisthe case, photogrammetry has only been applied to oil-film interferometry in a few cases (e.g., Zilliac [1]), and an in-depth discussion of its importance has not been presented. The use of photogrammetry to facilitate oil-film interferometry measurement of skin friction is the focus of this paper. The theory behind photogrammetry is presented, and specific issues related to oil-film interferometry are discussed. Specific examples of oil-film interferometry measurements that include photogrammetry are then provided. From these example cases, it is clear that photogrammetry reduces and quantifies geometric/imaging bias errors, and it substantially simplifies the handling of geometry in the analysis process. With uncertainty analyses of oil-film interferometry indicatingaccuraciesof2–3%whenperformedwithcare[2,3],these geometric/imaging bias errors become increasingly important if that level of accuracy is to be maintained in complex geometries. Althoughaparticularanalysismethodfordeterminingthewallshear stress from the fringe patterns obtained using oil-film interferometry isusedheretodemonstratetheapplicationofphotogrammetry,itwas simply chosen for convenience. The photogrammetry method discussed here can be used by other oil-film interferogram-analysis approaches to improve the geometry handling.

Experimental Investigation of Separation ControlPart 1: Baseline and Steady Suction

Low-speed flow separation over a wall-mounted hump, and its control using steady suction, were studied experimentally in order to generate a data set for the development and evaluation of computational methods. The baseline and controlled data sets comprised time-mean and unsteady surface pressure measurements, flowfield measurements using particle image velocimetry, and wall shear stress obtained via oil-film interferometry. In addition to the specific test cases studied, surface pressures for a wide variety of conditions were acquired for different Reynolds numbers and suction rates. Stereoscopic particle image velocimetry and oil-film flow visualization indicated that the baseline time-averaged separated flowfield was two-dimensional. With the application of control, mild three-dimensionality was evident in the spanwise variation of pressure recovery, reattachment location, and spanwise pressure fluctuations.

Skin Friction Measurements on the NASA Hump Model

The skin-friction distribution on a wall-mounted hump model has been obtained using oil-film interferometry. This effort is part of a larger study to provide validation cases for simulations of unsteady flows. The challenges of using oil-film interferometry on this model, including model curvature and close camera proximity, are discussed. Skin-friction measurements are obtained over most of the hump model, including especially high-quality measurements in the separated and reattachment regions. These results highlight the method’s ability to capture a wide range of skin friction including measurements in reverse-flow and high-gradient regions. The wall skin-friction data are shown to complement other experimental data, and the use of independent skin-friction measurements for scaling in wall-bounded flows is emphasized. A comparison with results from several computational simulations of the same flow is presented. The comparison indicates that, for the most part, the computations accurately predict the skin-friction ahead of separation, but fail to predict the reattachment point correctly, and thus the comparison in the separated and recovery regions of the flow is poor. The ability of the skin-friction measurements to pinpoint regions where the computation performs poorly in the near-wall region is also presented. From these results, it is evident that independent skin-friction measurements should be a part of all validation experiments conducted in wall-bounded flows.

Measurement and Calculation of Turbine Cascade Endwall Pressure and Shear Stress

The complex three-dimensional fluid flow on the endwall in an axial flow turbine blade or vane passage has been extensively investigated and reported on in turbomachinery literature. The aerodynamic loss producing mechanisms associated with the endwall flow are still not fully understood or quantitatively predictable. To better quantify wall friction contributions to endwall aerodynamic loss, low Mach number wind tunnel measurement of skin friction coefficients have been made on one endwall of a large scale cascade of high pressure turbine airfoils, at engine operating Reynolds numbers. Concurrently, predictive calculations of the endwall flow shear stress have been made using a computational fluid dynamics (CFD) code. Use of the oil film interferometry skin friction technique is described and applied to the endwall, to measure local skin friction coefficients and shear stress directions on the endwall. These are correlated with previously reported measured local endwall pressure gradients. The experimental results are discussed and compared to the CFD calculations, to answer questions concerning endwall aerodynamic loss predictive ability.